Devices using a piezoelectric phenomenon have been used in various fields. In the progress of miniaturization and power saving of portable equipment, the application field of surface acoustic wave (SAW) devices as RF filters or IF filters used for the above equipment is being enlarged. Enhancement of the design and producing technologies of SAW filters have satisfied user's strict requirements to specifications. However, as the frequencies being used are shifted to a higher frequency band, the enhancement of the characteristics is approaching to its upper limit, so that great technical innovation has been required for both of the microstructure of electrodes to be formed and securement of stable output.
Further, a thin film bulk acoustic resonator (hereinafter referred to as “FBAR”), and stacked thin film bulk acoustic resonators and filters (hereinafter referred to as “SBAR”) using the thickness vibration of piezoelectric thin film are each constituted of a thin film mainly composed of piezoelectric element and electrodes for driving the thin film on a thin support film provided on a substrate so that they can perform fundamental resonance in gigahertz band. When a filter is constructed by FBAR or SBAR, the filter can be formed in a remarkably compact size, and also it can be operated with low loss and in a broad band. In addition, it can be manufactured integrally with a semiconductor integrated circuit. Therefore, it is expected that FBAR and SBAR will be applied to future ultraminiature portable equipments.
A piezoelectric thin film resonator such as FBAR or SBAR applied to a resonator, a filter, or the like, using such elastic wave is produced as follows.
By using various thin film forming methods, a base film comprising a dielectric thin film, a conductive thin film or a stacked film of the dielectric thin film and the conductive thin film is formed on the surface of a single crystal semiconductor substrate of silicon or the like, on the surface of a substrate constructed by forming polycrystalline diamond film on silicon wafer, or on the surface of a substrate of constant modulus metal such as elinvar or the like. Further, a piezoelectric thin film is formed on the base film, and a desired upper structure is formed. After each film is formed or after all the films are formed, each film is subjected to physical processing or chemical processing to perform micro-fabrication and patterning. After a portion of the substrate located below the oscillation portion is removed by anisotropic etching based on a wet process to form a suspended structure, the resultant product is separated every device to obtain piezoelectric thin film resonators.
For example, one of methods of producing a piezoelectric thin film resonator known heretofore is a method for forming a via hole by forming a base film, a lower electrode, a piezoelectric thin film and an upper electrode on an upper surface of the substrate, and then removing a substrate portion under a portion which will act as an oscillating portion from a lower surface side of the substrate (for example, refer to Pat. Document 1: JP(A)-58-153412 and Pat. Document 2: JP(A)-60-142607). If the substrate is made of silicon, a part of the silicon substrate is etched away from the backside thereof by using a heated KOH water solution, thereby forming via hole. Thus, there can be produced a resonator having such shape that the edge of the structure in which a piezoelectric thin film is sandwiched between a plurality of metal electrodes is supported at the front surface side of the silicon substrate by the silicon substrate at a portion surrounding the via hole.
However, since etching is advanced in parallel with a (111) plane if wet etching using alkali such as KOH, etc. is performed, the etching is advanced obliquely at inclination angle of 54.7° with respect to the front surface of the (100) silicon substrate, and a distance between the adjacent resonators must be set remarkably far. For example, a device having a planar size of about 150 μm×150 μm and constructed on a silicon wafer having a thickness of 550 μm needs a backside etching hole of about 930 μm×930 μm, and a distance between centers of the adjacent resonators becomes 930 μm or more. This disturbs the integration of the piezoelectric thin film resonators. Further, a metal electrode for connecting the adjacent piezoelectric thin film resonators becomes long, and also its electric resistance increases. Therefore, an insertion loss of the piezoelectric thin film device produced by combining a plurality of the piezoelectric thin film resonators becomes remarkably large. A large via hole of an opening size of 930 μm is not only easy to be damaged, but also an acquired quantity of the final product, that is, a yield of the piezoelectric thin film device on the substrate is limited, and a region of about 1/15 of the substrate can be used for a device production. On the other hand, it is considered to form a large via hole bridging over a plurality of resonators. However, the via hole is further increased in size, a strength of the device is remarkably reduced, and the resonators are further easy to be damaged.
A second method of conventional art for producing a piezoelectric thin film resonator such as FBAR, SBAR, etc. applied to the piezoelectric thin film device is to form an air bridge type FBAR device (for example, refer to Pat. Document 3: JP(A)-2000-69594, Pat. Document 4: JP(A)-2002-509644 and Pat. Document 5: JP(A)-2003-32060). Normally, a sacrificial layer is formed, and then a piezoelectric thin film resonator is produced on this sacrificial layer. The sacrificial layer is removed at the end or near the end of the process, and a space for the oscillation is formed. Since the overall processing is executed on an upper surface side of the substrate, this method does not need an alignment of a pattern on both the surfaces of the substrate and an opening of a large area at the lower surface side of the substrate.
The Pat. Document 3 describes a construction of an air bridge type FBAR/SBAR device, and a method of producing it using a phospho-silicate glass (PSG) as a sacrificial layer.
However, this method requires long and complicated steps. That is, in this method, after a series of steps of forming a cavity on an upper surface of a substrate by etching, depositing a sacrificial layer on an upper surface side of the substrate by a thermal CVD (Chemical Vapor Deposition) method, planarization and smoothing of the upper surface of the substrate by a CMP (Chemical Mechanical Polishing) method, and depositing and forming a lower electrode, a piezoelectric element and an upper electrode on the sacrificial layer, a via hole penetrating to the sacrificial layer is opened, a piezoelectric laminated structure formed on the upper surface side of the substrate is protected with a resist or the like, and the sacrificial layer is removed from the cavity by permeating an etchant through the via hole. In addition, the number of masks used for forming the pattern is remarkably increased. Since the manufacturing steps are long and complicated, the method itself increases the cost of the device as well as reduces a yield of the product, and further the device becomes highly costly.
The Pat. Document 4 describes a construction of an air bridge type FBAR/SBAR device, and a method of producing it. In this method, a metal or a polymer is used as sacrificial layer, and a space for oscillation is formed in a relatively simple process without the steps of forming a cavity on an upper surface of a substrate and planarization of the upper surface of the substrate by CMP.
However, in order to form the space for oscillation excellently without contact of the piezoelectric laminated structure having a size of about 150 μm×150 μm with the substrate, the thickness of the sacrificial layer needs about 2000 nm. When a metal of 2000 nm is deposited, the surface roughness of the sacrificial layer is deteriorated by a growth of particle of metal crystal. When the piezoelectric laminated structure is formed on this sacrificial layer, a decrease in an electromechanical coupling factor Kt2 associated with a decrease in a crystal orientation of the piezoelectric thin film itself, and a decrease in a resonant sharpness Q associated with an increase in a surface roughness of the piezoelectric laminated structure itself, occur, and production of the piezoelectric thin film device having good characteristics becomes difficult. It is described that a polymer is used as the sacrificial layer, but in order to form the piezoelectric thin film having good crystal orientation, it is normally necessary to deposit the piezoelectric thin film at a temperature of 300° C. or higher in high vacuum. Thus, there is a problem in stability of the polymer. Further, since a bend of 2000 nm arises at ends of the piezoelectric laminated structure, there is a severe problem of remarkable deterioration of reliability by cracking or a decrease in strength of the piezoelectric thin film.
In the Pat. Document 5, it is described that a construction of an air bridge type FBAR/SBAR device, and a method of producing it. This method does not require steps of forming a cavity on an upper surface of a substrate and planarization the upper surface of the substrate by the CMP and can reduce a bend at ends of a piezoelectric laminated structure.
However, in this method, the sacrificial layer is etched in advance with a first etchant, and further a second etchant is introduced by using the resultant gap to etch a supporting film. Thus, a space for oscillation is formed. Accordingly, it is necessary to form the device with use of a material durable to two types of etchants, which restricts the material to be used and which makes the process complex, thereby increasing the production cost. Further, as the sacrificial layer, substances such as a magnesium oxide, a zinc oxide, etc., are used. When these substances form a film by a vapor-depositing method, etc., the surface roughness of the sacrificial layer is large, and crystal orientation of the lower electrode and the piezoelectric thin film formed on the sacrificial layer is deteriorated. For example, in the case of the magnesium oxide thin film having a thickness of 50 nm, its surface roughness (RMS variation in height) becomes normally 10 nm or more.
Since FBAR and SBAR obtain a resonance by propagation of elastic wave generated by a piezoelectric effect of a piezoelectric element in the piezoelectric laminated structure, the characteristics of the device are greatly affected by a crystal quality of a lower electrode, a piezoelectric thin film, an upper electrode, or the like on the substrate as well as an accuracy of forming a space for oscillation. Further, when the bend of the piezoelectric thin film is large, the strength of the piezoelectric thin film decreases, and reliability remarkably decreases. Therefore, it becomes remarkably difficult to stably obtain the piezoelectric thin film device having excellent characteristics and high reliability.
By such a reason, a piezoelectric thin film device which performs sufficient performance in gigahertz band has not been obtained. Therefore, establishment of a method of producing a piezoelectric thin film device having excellent characteristics and high reliability with a simple process and realization of the piezoelectric thin film device produced by the method are strongly desired.